1. Field of the Invention
This invention relates to a suspension system for a vehicle, and in particular, to an active suspension system.
2. Description of Related Art
A conventional active suspension is disclosed in, for example, Japanese Patent Publication No. 62-96113, laid open to the public on May 2, 1987, a European Patent Application, laid open to the public on Aug. 1, 1984 under a publication number 0114757, and U.S. Pat. No. 4,371,182 issued on Feb. 1, 1983. In the active suspensions as disclosed in the publications referred to above, a cylinder device is provided for each vehicle wheel member between a vehicle body member and a vehicle wheel member. By controlling a supply of the hydraulic fluid for the cylinder device, the hydraulic fluid in the cylinder device can be changed so that the vehicle suspension characteristics are changed.
In the active suspension system, a hydraulic pressure is detected and a warping of the vehicle body is computed based on the hydraulic pressure detected. A warp control is accompished by controlling a hydraulic fluid supply for the cylinder device.
In addition, a vehicle height displacement is detected and a supply of the hydraulic fluid is controlled for the cylinder device to get a target value of the vehicle height displacement (a vehicle height displacement control).
The warp control and the vehicle height displacement control are carried out independently. Therefore, both of the controls must be coordinated with each other for improving the suspension property.